The invention concerns the transmission of multimedia type contents via a radio communication network implementing long period time interleaving at the level of the physical layer, and more particularly the time interleaving and deinterleaving, within such networks, of units sometimes referred to as interleaving units (or IU, in particular in DVB-SH and SDR type networks) defined based on multimedia content data packets to be transmitted.
The term “multimedia content” as used herein refers to sets of data of one and the same type, such as, for example, television or radio programs, data, or video files.
Furthermore, the term “radio communication network” as used herein shall mean any kind of satellite and/or terrestrial radio network implementing long period time interleaving at the level of the physical layer with a view to broadcast of multimedia content. This could for example be an SDR (“Satellite Digital Radio”) network or a hybrid DVB-SH type network (DVB-H (“Digital Video Broadcasting—Handhelds”) radio network type but adapted to S-band satellite links (approx. 2.20 GHz)—mobile television via satellite and terrestrial relays). Generally speaking, the invention concerns any network likely to suffer from significant signal fading.
As is known to a person skilled in the art, when certain radio (or Hertzian) signals propagate, they can become subject to multi-path interferences, attenuations and transient interceptions. This is frequently the case when signals propagate in a satellite channel. In order to limit the impact of interference, attenuations and interceptions, it is possible to implement, on the transmission side, a time period interleaving technique at the level of the physical layer, i.e. at the level of the content data packets to be transmitted. This is in fact what is done in DVB-SH type networks.
In the networks concerned by the invention, the length (or time constant) of the time interleaving at the level of the physical layer is (very) large as compared to the time constant used in the service layer. It can reach up to 10 seconds.
It is hereby reiterated that the TDMA (Time Division Multiple Access—time multiplexing) frames delivered by the service layer consist in a predefined number (e.g. 9) of bursts of MPEG type packets, which are respectively associated to services. These packets are encoded using an FEC (Forward Error Correction) type error correction technique, as well as in addition (possibly) MPE-FEC (Multi-Protocol Encapsulation—FEC), to generate codewords. Each burst of codewords is then broken down into interleaving units (IUs). The interleaving units of the different bursts of a frame are then distributed (or spread) by the interleaver over the entire time period interval occupied by a time interleaving period. The interleaving unit is the smallest set of bits (arising out of a content packet) that is processed by an interleaver.
Therefore, if it is desired to reconstitute, in a receiver terminal, all bursts of an interleaved frame, it is necessary to store in a an external memory all the interleaving units contained in a time interleaving period, which can require a greater amount of memory. By way of example and not of limitation, in the case of a type DVB-SH network using 16QAM (or respectively QPSK) modulation and a time interleaving period of 8 seconds, the receiver terminal must provide an external memory of approximately 256 MB (Megabytes) (respectively 128 MB) to store the data contained in a time interleaving instance. This need for a large capacity memory is a significant constraint for the manufacture of receiver terminals at a lower cost and complexity.
It would of course be possible to carry out the time interleaving on a layer located at a higher level than the physical layer. Such higher level layers do in fact enjoy the advantage of non-quantification of data bits and lower coding rates enabling time interleaving of a length similar to that obtained at the level of the physical layer, but requiring much less memory capacity (typically 10 times less). However, the FEC error correction technique as well as, additionally (possibly) MPE-FEC, which is applied before the time interleaving step, is substantially more effective when applied at the level of the physical layer, with an identical spectral efficiency. Moreover, interleaving at the level of the MPE-FEC layer, which is located above the physical layer, is distinctly less efficient than that of the said physical layer, and signaling of this MPE-FEC layer is no longer usable due to the interleaving time period of the physical layer.
The purpose of the invention is therefore to improve the situation.